Free Compound Interest Calculator
Enter a starting amount, monthly contribution, interest rate and time period to see how compound interest grows your money. The earlier you start, the more dramatic the result.
Compound Interest Calculator
Mr B says: The earlier you start, the less you need to contribute. Time is doing most of the work — a dollar invested at 25 is worth dramatically more than a dollar invested at 45.Why time matters more than the amount you start with
Compound interest is interest earned on interest, not just on your original deposit. Each year's growth becomes part of the balance that earns the following year's growth, which is why the curve gets steeper the longer money is left invested. The practical implication is that starting age matters more than most people expect — someone who invests $200 a month from age 25 will typically end up with substantially more at retirement than someone who invests $400 a month starting at 35, purely because of the extra decade of compounding.
Contribution size versus time in the market
It's tempting to think you can make up for a late start by contributing more each month. That's true up to a point, but the maths rarely works out as well as people hope — doubling your monthly contribution doesn't double your end result if you've lost ten years of compounding along the way. This calculator is a useful way to test that directly: run the numbers for starting now versus starting in two years and see the actual gap in dollars, not just in theory.
What this calculator assumes
The projection assumes a constant rate of return applied consistently over the full period, which real markets never actually deliver — returns are lumpy year to year even if they average out over decades. Treat the result as a reasonable long-run estimate for planning purposes, not a guarantee. A more conservative rate assumption gives you a more reliable floor to plan around than an optimistic one.
Common questions